The invention is directed to a method and apparatus for thermo-forming sheet material, and particularly to a method and apparatus for corrugating amorphous materials having low melt strength at forming temperatures.
High melt strength sheet materials may be corrugated by drawing or extruding heated sheet material through a calibrator having the profile corresponding to the desired final shape. Low melt strength materials may not be readily formed by this method because pulling or drawing forces may exceed the strength of the material above the glass transition temperature. If the material is extruded below the glass transition temperature, the pulling force may be excessive and, in addition, the sheet material may be damaged by the calibrator or extruder friction. One way to reduce the friction is to use rollers to deform the sheet material. However, certain materials such as polycarbonates have the tendency to shrink back from the deformed shape as the material cools below the glass transition temperature.
The use of deforming rollers and a calibrator has also proved ineffective because the materials tend to pull back from the deformed shape before calibration can be effected. Thus, although the rollers prevent initial damage, the calibrator friction causes damage.
A need therefore exists for a method and apparatus for corrugating sheet material having low melt strength at forming temperatures.